This invention relates to a method for forming a through-hole on a mask, especially a stencil mask of an electron beam stepper, used for forming a desired pattern by projection on a semiconductor wafer or the like.
Conventionally, as a method for forming a through-hole on a stencil mask such as a silicon thin film, sputtering etching or gas assisted etching using a focused ion beam device as shown in FIG. 5 is carried out. Specifically ions are drawn out from an ion source 10 such as liquid gallium, accelerated by focusing into a beam through an ion optical system 11, and irradiated while scanning a sample on a sample stage 15 by deflecting so that etch processing is carried out. However, a through-hole in this method becomes a cone-shaped hole due to etching through a wide area on the surface of the side where a focused ion beam 12 is irradiated. If a square region is set to be scanned by beam irradiation, the hole is formed in an inverted pyramid shape as shown in FIG. 4B, which shows a view from the side where an ion is irradiated and FIG. 4A shows a cross-section thereof. Even if the scanning region shown in the drawing is set and an etching process is carried out a through-hole H then corresponds to the scanning region at the back surface, and the opening at the side where an ion is irradiated becomes a sloping S shape. This is because if ions focused into a beam by an optical system 11 are irradiated, in reality, it is a Gaussian distribution, so the periphery of the beam is also influenced by weak ion irradiation and this causes a side irradiated with a focused ion beam to be exposed to the irradiation for a long time resulting up to beyond the set region. Theoretically speaking, it is difficult to excavate a deep hole with a sputtering process using a focused ion beam 12 involving re-attachment. That is, there is a limit to irradiating a focused ion beam 12 into the deep inner part of a small hole and eliminating sputtered material from outside of the hole without re-attachment. The limit is such that a 0.3 xcexcm through-hole (aspect ratio:5) is formed in a thin film having a thickness of 1.5 xcexcm.
It is difficult to process a hole of much higher aspect ratio even if a gas assisted etching technique where an assisted ion beam 12 is irradiated while gas is sprayed from a gas gun 14 is employed to process in order to avoid re-attachment. Further, the slope of the opening surface has nonuniformity of a sample thin film thickness which cause a problem that, when using an electron beam stepper or the like as a stencil mask, electron diffusion becomes non-uniform and this is not suitable for a stencil.
The object of the present invention is to provide a processing method capable of forming a vertical edge through-hole on a thin film of a stencil mask such as an electron beam stepper, the hole having a small diameter with respect to the thickness of the thin film, and this kind of stencil mask.
In this invention, a method for forming a vertical edge submicron through-hole comprising the steps of; forming a large hole, in a thin film sample, with a diameter larger than a design size of a through-hole, with a bottom having thickness close to the design size remaining, by etching using a focused ion beam device; forming a through-hole having the design size in the bottom section by focused ion beam etching; and backfilling the large hole to the design size by deposition using a focused ion beam device, and another forming method is further adopted comprising the step of finishing the inner surface of the through-hole by gas assisted etching using a focused ion beam device.